1. Technical Field of the Invention
This invention relates generally to electromagnetic transducers such as audio speakers, and more specifically to a shorting ring which also serves as an axial alignment fixture within the motor structure.
2. Background Art
FIG. 1 illustrates a conventional internal magnet geometry motor structure 10 for use in an electromagnetic transducer such as an audio loudspeaker. The motor structure includes a magnetically conductive cup style yoke 11 which includes a cylindrical portion 12 and a back plate portion 13. A magnetically conductive cup spacer 14 may be present to increase bottoming clearance, and it may be magnetically coupled to or integrally formed with the cup. A permanent magnet 15 is magnetically coupled to the spacer, and a magnetically conductive top plate 16 is magnetically coupled to the permanent magnet. The top plate and the cylindrical portion of the cup define a magnetic air gap 17 within which the electromagnetic transducer's voice coil (not shown) travels.
It is important, for correct operation of the transducer, that the various components remain coaxially aligned, as illustrated. If one or more components become misaligned, the acoustic performance may suffer, and the loudspeaker may even be damaged, such as by the voice coil assembly striking or rubbing on the motor structure components.
Conventionally, coaxial alignment is initially provided during manufacturing, by the use of a jig or fixture which is used to align the components for gluing, and which is subsequently withdrawn after the glue sets. The need to withdraw the fixture prevents the manufacture of some motor structure geometries, and adds to the manufacturing cost. Mistakes in fixturing cause reduction in manufacturing yield, raising the effective cost of correctly manufactured parts.
After manufacturing, coaxial alignment of most components is maintained by the integrity of the glues which are used to couple the magnet to the cup, and to couple the top plate to the magnet. If other, more mechanical means are employed, such as bolts, they raise the manufacturing cost without providing any acoustic or other performance advantage.
What is needed, then, is an improved fixture that becomes a functional component improving the motor structure's performance, that does not significantly raise the manufacturing cost, that improves the long-term reliability of the motor structure, and that provides a mechanical alignment which is not dependent upon e.g. glue reliability.
Many manufacturers subject speaker drivers to drop testing. Glue layers increase magnetic reluctance in the magnetic circuit in proportion to the thickness of the glue layers. What is further desirable, then, is an improved motor structure which requires less glue and therefore thinner glue layers, while having improved resistance to sheer failure in drop testing.